For example, a digital image coming from a digital photographic apparatus consists of a set of N×M elementary image units or pixels, where N is the height of the image and M its width. This image is coded before being stored in memory. The initial data, that is to say the information representing the pixels of the image, are organized in a bi-dimensional table accessible for example line by line.
A digital image generally undergoes a transformation prior to its coding. Likewise, when a coded digital image is decoded, the image undergoes a reverse transformation. The transformation may consist of applying a filter to all or part of the digital image.
A filter may be seen as a convolution product between the image signal and a predetermined vector making it possible, for each pixel of the region to which it is applied, to modify its value according to values of the adjoining pixels, to which coefficients are allocated.
The coding technique described in the patent document FR-A-2 889 382 makes it possible to filter the signal prior to a compression, by orienting the filter along certain directions, for each pixel, with a view to reducing the dynamic range of the signal generated and thus to increase the compression of the signal.